The present invention generally relates to an engine speed control system and, more particularly, to a control system for controlling the engine speed during idling to a predetermined engine speed.
In spite of various conditions surrounding the automobile internal combustion engine such as change in load applied to the engine, change in density of the atmospheric air, the degree of lapping of the engine, and variation in the engine tolerances or the like, there is a requirement for maintaining the engine speed at a predetermined value. Stable engine speed is becoming important in that the engine speed during idling is required to be made lower to reduce fuel consumption and the emission of noxious components of exhaust gases to the atmosphere during idling of the engine.
One type of known automobile idle control device includes an electrically operated actuator for adjusting the engine idling speed by varying the effective area of a bypass air passage leading to a fuel intake passage at a position downstream of the throttle valve. The idle control device also has a speed detector for detecting the engine idling speed and a control circuitry for receiving an output signal from the speed detector and operating the actuator when the engine speed during idling deviates from the predetermined value, thereby correcting the idling speed to the predetermined value.
For example, the Japanese Patent Publication No. 49-40886, published on Nov. 6, 1974, discloses an engine idle control device comprising an electrically operated actuator which varies the effective area of a bypass air passage having one end communicated to the atmosphere and the other end communicated to the fuel intake passage at a position downstream of the throttle valve; a tachometer for detecting the engine speed and generating an electric signal indicative of the actual engine speed detected; a reference voltage generator for generating a reference voltage corresponding to a desired or predetermined engine idling speed; a controlling circuit having a comparator for comparing the electric signal from the tachometer with the reference voltage; and means for operating the actuator to adjust the engine idling speed to the predetermined value when the difference is created between the electric signal from the tachometer and the reference voltage.
The Japanese Utility Model Laid-open Publication No. 54-73417, laid open to public inspection on May 25, 1979, discloses an apparatus similar to that disclosed in the first mentioned Japanese publication. To more accurately control the engine idling speed to the predetermined value, the actuator used in this Japanese publication is operated with a continuously repeating pulse signal so that the duty cycle of the actuator can be controlled. It is to be noted that the duty cycle referred to above is the ratio, expressed as a percentage, of the time during which an electric current is supplied to the solenoid of the actuator, relative to the unit time and that 0% duty cycle and 100% duty cycle stand respectively for the closure and full opening of the actuator.
According to the prior art, even during the stable idling of the engine with no change in the ambient conditions under which the engine is operated, the engine idling speeds tends to deviate from the predetermined value. To solve this problem, correction of the engine speed during idling performed when the ambient conditions under which the engine is operated changes, for example, by reason of an automobile air-conditioner being operated, to the predetermined engine speed requires a relatively long time. In other words, the system response to a change in engine speed is slow. Conversely, if attempt is made to improve the system response characteristic, the stability of the engine speed is adversely affected.
On the other hand, the U.S. Pat. No. 4,137,877, patented Feb. 6, 1979, discloses an air-fuel ratio control circuitry which may be employed in the practice of the present invention.